1. Field of the Invention
The present invention is directed generally to a switched power pack with a protective device for limiting output voltage and, more particularly, to a device including a drive means for driving a switching transistor with which a DC-voltage is periodically connected to a primary winding of a transformer, whereby the transformer has at least one secondary winding at which the output voltage is taken via a rectifier means.
2. Description of the Related Art
A circuit arrangement for a switched power pack is disclosed in German Published Application No. 33 41 074 A1. For protection against excessively high-voltage at the secondary side of the switched power pack, a thyristor is provided which periodically inhibits the switching transistor when excessively high current is present at the primary side.
It is known to stabilize the output voltage of switched power packs with a control loop. For example, in Siemens Produkinformation "ICs Fuer Die Unterhaltungselektronik, SNTs", Edition of August '87 is disclosed the acquisition of the output quantity or the deviation occurring via a control winding at the transformer of the switched power pack. This can also occur by the use of an opto-coupler. The switching transistor is driven dependent on the identified output quantity or deviation via a control and monitoring means.
When the control loop malfunctions in the known circuit arrangements, however, then the output voltage of the switched power pack can rise to unacceptable values. Given a rise of the output voltage of from 20 through 40%, the signal processing stages of the control and monitoring means remains functional since their supply voltages are usually re-stabilized. In other words, the supply voltage is regulated to stay within acceptable limits so that the device continues to operate.
This presents a particular problem when the switched power pack is used as a power supply for television sets or other picture tube devices. When, in particular, the non-stabilized supply voltage of the horizontal sweep circuit (H-sweep) rises by this percentage, then the high-voltage at the picture tube of the television set also rises in the same ratio, for example, from 25 to 35 kv. The television picture may seen slightly enlarged but remains sharp to the observer, so that the set usually continues to be used. However, the acceptable limit values of x-rays emitted by the television set are dangerously exceeded. The x-ray regulations in some individual countries, therefore, prescribe reliable circuits which prevent the increase of the high-voltage at the picture tube of the television set. For example, in the Federal Republic of Germany, the limit value of admissible x-radiation from a television set lies at 30 kv.
The main problem lies in that then bipolar transistors are usually employed in the switched power packs and horizontal sweep circuits of the line output stage, which exhibits a very large safety margin in the dielectric strength. For example, a bipolar transistor BU 508 is used in switched power packs and is operated with about 700 volts, and in a 90.degree. line output stage is operated with about 950 volts. The limiting data for the bipolar transistor BU 508 is 1500 volts and the destruction limit lies at about 1700 through 1800 volts, which means that the output stages almost never fail due to over-voltage. Instead, when a malfunction occurs which affects the control voltage, dangerously high output voltages are generated by the power pack and, thus, excessively high voltages at the picture tube and resulting dangerously high x-radiation levels occur.